On (2,3)-generated groups Maxim Vsemirnov Steklov Institute of - PowerPoint PPT Presentation

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z On (2,3)-generated groups Maxim Vsemirnov Steklov Institute of Mathematics at St. Petersburg Group Theory Conference in honor of V. D. Mazurov Novosibirsk, July 20, 2013 M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Outline Definitions and motivations 1 (2,3)-generated finite (simple) groups 2 (2,3)-generated classical groups over Z 3 M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Outline Definitions and motivations 1 (2,3)-generated finite (simple) groups 2 (2,3)-generated classical groups over Z 3 M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z (2,3)-generated groups Definition A (2,3)-generated group is a group generated by an involution and an element of order 3. Definition An ( m , n ) -generated group is a group generated by two elements of order m and n , respectively. Why is the (2,3)-generation problem interesting? M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Why do we look at ( 2 , 3 ) ? The modular group PSL 2 ( Z ) is isomorphic to the free product of two cyclic groups, C 2 and C 3 . �� 0 � � 0 �� − 1 − 1 PSL 2 ( Z ) = , ≃ C 2 ∗ C 3 . 1 0 1 − 1 Thus, apart from { 1 } , C 2 , and C 3 , all quotients of PSL 2 ( Z ) are exactly the ( 2 , 3 ) -generated groups. M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Comparison with PSL n ( Z ) , n ≥ 3. Remark The normal subgroup structure of PSL 2 ( Z ) differs dramatically from the normal subgroup structure of PSL n ( Z ) , n ≥ 3. Namely, for n ≥ 3, any subgroup of finite index in PSL n ( Z ) is a so-called congruence subgroup, i.e., contains the kernel of PSL n ( Z ) �→ PSL n ( Z / m Z ) for some m . In contrast, PSL 2 ( Z ) contains many noncongruence normal subgroups (even of finite index). M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Outline Definitions and motivations 1 (2,3)-generated finite (simple) groups 2 (2,3)-generated classical groups over Z 3 M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Two different approaches There are two different groups of methods in this area: constructive, i.e., when the corresponding generators are given explicitly; non-constructive, e.g., probabilistic, when only existence theorems are known (usually require a good knowledge of the characters and maximal subgroups). Constructive methods can be also applied to infinite groups. M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Known results For any m , PSL 2 ( Z / m Z ) is (2,3)-generated (trivial) 1 A n , n ≥ 4, are ( 2 , 3 ) -generated except A 6 , A 7 , and A 8 2 (Miller, 1901). Sporadic groups are (2,3)-generated except M 11 , M 22 , M 23 , 3 and McL (Woldar, 1989) 2 B 2 ( 2 2 k + 1 ) are not (2,3)-generated (trivial, since they do 4 not contain elements of order 3) Other exceptional Lie groups are ( 2 , 3 ) -generated (Malle, 5 1990, 1995, Malle and Lübeck, 1999) M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Classical groups and the (2,3)-generation Negative results for certain small groups: PSL 2 ( 9 ) ≃ Sp 4 ( 2 ) ′ ≃ A 6 , PSL 4 ( 2 ) ≃ A 8 , PSL 3 ( 4 ) , PSU 3 ( 9 ) . For n large enough, SL n ( q ) are (2,3)-generated; Tamburini, J. Wilson (1994–1995), n ≥ 14; Di Martino, Vavilov (1994–1996) for n ≥ 5, q � = 3, q � = 2 k . PSp 4 ( p k ) are (2,3)-generated if p � = 2 , 3 (Di Martino and Cazzola, 1993). PSp 4 ( p k ) are not (2,3)-generated if p = 2 , 3 (Liebeck and Shalev, 1996). Almost all classical groups are ( 2 , 3 ) -generated (Liebeck and Shalev, 1996). M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Probabilistic methods Theorem (Liebeck, Shalev, 1996) Let G run through some infinite set of finite classical groups, G � = PSp 4 ( p k ) . Then | G |→∞ Prob ( x 2 = y 3 = 1 and G = � x , y � ) = 1 . lim Moreover, the result remains true if we fix the field and let the rank tend to infinity; if we fix the type and let the size of the field tend to infinity. M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z New examples of non (2,3)-generated groups Theorem (V., 2011) PSU 5 ( 4 ) is not ( 2 , 3 ) -generated. | PSU 5 ( 4 ) | = 13 , 685 , 760 = 2 10 · 3 5 · 5 · 11 A sketch of the proof. 1. dim ker ( x − 1 ) = 3 and y ∼ diag ( 1 , ω, ω, ω − 1 , ω − 1 ) , ω 2 + ω + 1 = 0.     0 1 0 0 c 1 0 a 0 b − c − 1 1 0 0 0 0 0 0 0         − 1 x = 0 0 0 1 d , y = 0 1 0 0 ,         0 0 1 0 − d 0 0 0 0 − 1     0 0 0 0 1 0 0 0 1 − 1 for some a , b , c , d . M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z A sketch of the proof (cont.)   3 + a ac + bd − b a + b  = 0 , then 2. If det − 1 b + cd + c c − a − 1  1 − c + d 2 + d − 1 1 + d � x , y � has a 1-dimensional invariant space. 3. If � x , y � preserves a hermitian form then a = − d − d σ − 1, b = − c + c σ + d + d σ + d 2 + dd σ − 1. 4. There are 16 pairs of parameters ( c , d ) . For four of them, the group is defined over F 2 . For ten of them, det ( · · · ) = 0. For the remaining two, setting z = yx we have z 11 = x 2 = ( zx ) 3 = ( z 4 xz 6 x ) 2 = 1 , a well-known presentation of PSL 2 ( 11 ) . M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z New examples of non (2,3)-generated groups Theorem (Pellegrini, Tamburini Bellani, V., 2012) PSU 4 ( 9 ) is not ( 2 , 3 ) -generated. Theorem (V., 2012) Ω + 8 ( 2 ) , P Ω + 8 ( 3 ) are not ( 2 , 3 ) -generated. | PSU 4 ( 9 ) | = 3 , 265 , 920 = 2 7 · 3 6 · 5 · 7 8 ( 2 ) | = 174 , 182 , 400 = 2 12 · 3 5 · 5 2 · 7 | Ω + 8 ( 3 ) | = 4 , 952 , 179 , 814 , 400 = 2 12 · 3 12 · 5 2 · 7 · 13 | P Ω + M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Non (2,3)-generated finite simple groups PSp 4 ( 2 k ) 1 PSp 4 ( 3 k ) , in particular PSp 4 ( 3 ) ≃ PSU 4 ( 4 ) 2 2 B 2 ( 2 2 k + 1 ) 3 A 6 ≃ PSL 2 ( 9 ) ≃ Sp 4 ( 2 ) ′ , A 7 , A 8 ≃ PSL 4 ( 2 ) 4 PSL 3 ( 4 ) , PSU 3 ( 9 ) ≃ G 2 ( 2 ) ′ 5 M 11 , M 22 , M 23 , McL 6 PSU 5 ( 4 ) 7 PSU 4 ( 9 ) 8 Ω + 8 ( 2 ) , P Ω + 8 ( 3 ) 9 10 ? I strongly believe that the list is complete. M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z Outline Definitions and motivations 1 (2,3)-generated finite (simple) groups 2 (2,3)-generated classical groups over Z 3 M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z The main Theorem Theorem The groups SL n ( Z ) and GL n ( Z ) are ( 2 , 3 ) -generated precisely when n ≥ 5 M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z An overview of results SL 2 ( Z ) is not ( 2 , 3 ) -generated as it contains no non-central involution. SL 4 ( Z ) and GL 4 ( Z ) are not ( 2 , 3 ) -generated as SL 4 ( 2 ) = GL 4 ( 2 ) ≃ A 8 is not (Miller, 1901) SL 3 ( Z ) and GL 3 ( Z ) are not ( 2 , 3 ) -generated (Nuzhin, 2001, Tamburini, Zucca, 2001). SL n ( Z ) and GL n ( Z ) are ( 2 , 3 ) -generated for n ≥ 14 (Tamburini, et al. 1994–1995, 2009) For SL 5 ( Z ) , GL 5 ( Z ) and SL 6 ( Z ) there are at most finitely many conjugacy classes of ( 2 , 3 ) -generators (Luzgarev, Pevzner, 2003, Vsemirnov, 2006). The groups SL n ( Z ) and GL n ( Z ) , n = 5 , . . . , 13 are ( 2 , 3 ) -generated (Vsemirnov, 2007–2009). M. Vsemirnov On (2,3)-generated groups

Definitions and motivations (2,3)-generated finite (simple) groups (2,3)-generated classical groups over Z An idea of the proof Two difficult problems: to guess the shape of ( 2 , 3 ) -generators; to show that they actually generate SL n ( Z ) . The main idea: show that � x , y � contains some generating set of SL n ( Z ) . For instance, one can show that � x , y � contains elementary transvections t ij ( α ) = I + α e ij , i � = j . M. Vsemirnov On (2,3)-generated groups